Monopulse radar tracking systems have traditionally relied on complex antenna and phasing structures to produce and receive radar signals. In contrast, the present invention provides a simple antenna structure that takes advantage of the propagation and superposition properties of circularly polarized (CP) waves.
Accordingly, before describing the invention in more detail, it will be useful at this point to briefly review monopulse radar tracking systems and the generation and superposition properties of CP waves.
The brief overview of the monopulse radar tracking system and the generation and superposition properties of CP waves is described with reference to the accompanying Figures in which:
FIG. 1 which is a block diagram of a traditional monopulse radar tracking system apparatus; and
FIGS. 2(a), 2(b), 2(c) and 2(d) are diagrammatic representations of a metal post-probe apparatus for generating CP waves, wherein for ease of interpretation, FIGS. 2(a), 2(b), 2(c) and 2(d) additionally show the orientation of the electric field vectors generated in the apparatus and the rotational directions of the resulting CP waves.
(a) Monopulse Radar Tracking Systems
The purpose of a tracking system is to determine the location or direction of a target on a near-continuous basis. This data can then be used by a fire control system, to ascertain the target's motion and predict its future position.
One of the most accurate electronic scanning techniques for the purpose of target tracking is the monopulse (simultaneous) lobing technique. In the monopulse radar tracking technique, the range, bearing and elevation-angle of a target can be determined from a single pulse. FIG. 1 shows a simplified block diagram of a traditional four horn monopulse radar tracking system apparatus. Each pulse comprises four signals of equal amplitude. The four signals are radiated at the same time from four horns A, B, C and D that are grouped together in a cluster 2.
The monopulse radar tracking system is designed so that the signal from each horn can be distinguished from those of the other horns, for example, by using different polarizations for each horn's signal. A comparator circuit 4 continuously compares the amplitude (or phase) of the return echo signals received by each horn with those received in the other horns. The comparator circuit 4 comprises waveguides and four hybrid junctions called “magic tees” (6, 8, 10 and 12 in FIG. 1). Each junction receives two input signals and produces two output signals representing the sum and difference between the two input signals respectively. Accordingly junction 6 produces outputs (A+B) and (A−B) and junction 8 produces outputs (C+D) and (C−D). Similarly, junction 10 produces outputs Σo ((A+C)−(B+D)) and ΔEl ((A+D)−(B+C)) and junction 12 produces outputs Σ1 (A+B+C+D) and ΔAz ((A+B)−(C+D)).
In order to determine whether a target is present and determine its range, the beams from all four horns are summed (i.e. to generate the sum signal Σ1). The resulting beam has a single lobe. Consequently, the radar will receive a large return signal from a target centered within the beam.
If a target is detected, the return signals from the four horns are combined by junctions 6, 8, 10 and 12, to produce broadside difference patterns. These difference patterns are characterised by the presence of a broad peak and a sharp null. The ΔAz output from junction 12 is used to determine the azimuth of the target. The ΔEl output of junction 10 is used to determine the elevation of the target. If the target is located on the boresight axis, the amplitude of the target's return signal will be equalized in all four horns (i.e. the target will be located in the null region). The radar system's tracking circuit and power drives use this principle to track the motion of a target by moving the horn cluster 2 in the direction which equalises the amplitude of the return signal in all four horns A, B, C and D.
Notwithstanding the accuracy advantages of the monopulse radar tracking technique over conventional mechanical scanning radar tracking techniques, the monopulse radar tracking system apparatus is typically bulky and expensive because it requires four independent (or partitioned) horns.
U.S. Pat. No. 6,281,855 describes a single radiating element antenna structure capable of producing monopulse summation and difference far field patterns. In effect, the antenna operates by electromagnetically creating conditions for four separate radiating apertures within a single physical aperture. More specifically, the antenna employs four individually fed dielectric rods inserted into the horn and symmetrically disposed along the horn's major axis. When excited, the dielectric rods cause the electric fields inside the horn to distort and become asymmetrical thereby producing the summation and difference far field patterns.
(b) Generation and Superposition Properties of Circularly Polarized (CP) Waves
General Properties of CP Waves
The polarization of an electromagnetic wave is defined by the shape and orientation of the tip of the E vector as it varies with time. Circular polarization is a polarization state where the perpendicular components of an electrical field are of equal magnitude and have a 90° phase difference, so that the tip of the electric field vector traces a circle on the plane that is perpendicular to the direction of wave propagation. When the tip of the electric field vector rotates in a clockwise direction, as viewed from the antenna, as time progresses a right-hand circularly polarized (RHCP) wave is generated. Similarly, when the tip of the electric field vector rotates in an anti-clockwise direction a left-hand circularly polarized (LHCP) wave is generated.
Whilst the electric field vector rotates in a circle in the plane perpendicular to the direction of wave propagation, along the propagation axis itself, the movement of the tip of the electric field vector describes a helix.
Generating CP Waves
A CP wave can be generated by passing a linearly polarized (LP) wave through a waveguide that contains an internal delay element positioned at 45° with respect to the LP wave. The components of the LP signal are thus decomposed into two orthogonal E vectors. Since the LP wave which passes through the delay element travels more slowly than through the waveguide, a phase difference is created between the portion of the wave which travels through the waveguide and the portion that travels through the delay element. If the waveguide and the delay element are of sufficient length a differential 90° phase shift can be induced between the two portions of the LP wave. Provided these are of equal magnitude then when these two portions of the LP wave are combined at the output of the waveguide, a circularly polarized signal is produced. The above delay-waveguide structure behaves like a low pass filter.
FIGS. 2(a), 2(b), 2(c) and 2(d) show an alternative apparatus for generating CP waves, wherein the apparatus behaves more like a high pass filter. Referring to FIG. 2(a), the apparatus comprises two metal posts 14 and 16 aligned at 180° to each other. Assuming that post 16 represents the 0° position, the apparatus further comprises a probe 18 positioned at 225°. If a voltage is applied to the probe 18, the resulting electric field resolves itself into two components, namely a vertical component Ev which is directed along the post 16 and a horizontal component EH which is directed perpendicularly to the post 16. The vertical component Ev must travel past the metal posts. However, since a field moves more slowly past the metal posts than through air, the vertical component Ev experiences a phase shift compared to EH. The metal posts are designed to ensure that this phase shift is 90°. Accordingly, in a manner akin to the above-mentioned waveguide-dielectric system, the wave produced from the output of the metal post-probe apparatus is circularly polarized. For the sake of clarity, the resulting wave is hereby defined to have a right-handed rotation (i.e. an RHCP wave)
FIGS. 2(b), 2(c) and 2(d) show a similar apparatus to that of FIG. 2(a). However, in the case of FIG. 2(b) the probe 18 is located at the 135° position (relative to post 14) and the resulting wave is an LHCP wave. In the case of FIG. 2(c) the probe 18 is located at the −45° position (relative to post 14) and the resulting wave is an LHCP wave with a 180° phase shift. Finally, in FIG. 2(d) the probe is located at the +45° position (relative to post 14) and the resulting wave is an RHCP wave with a 180° phase shift.
Superposition Properties of CP Waves
If an RHCP wave is combined with an LHCP wave, the result is an LP wave. Similarly, if an RHCP wave is combined with an LHCP wave with a 180° phase shift, the result is an LP wave with a 90° phase shift with respect to the case where no phase shifts are applied to either CP signal.